The present invention relates to a method for preparing a lithographic printing plate, and specifically to the plate-making method wherein a presensitized plate for use in making a lithographic printing plate (hereinafter possibly referred to as xe2x80x9cPS platexe2x80x9d) is developed by a specific developer. More specifically, the present invention relates to the plate-making method wherein a photopolymerizable presensitized plate for use in making a lithographic printing plate (hereinafter possibly referred to as xe2x80x9cphotopolymerizable PS platexe2x80x9d) is developed by a specific developer, which method never shows any reduction of developing characteristics due to the elapse of a long period of time and repeated use of the developer, can ensure a good ability to develop the non-image area (non-exposed area) of the lithographic printing plate, prevents the non-image area from scumming, does not damage the image area (exposed area) of the plate due to the development, ensures the formation of firm images and allows the achievement of high printing durability.
The developers widely used conventionally for developing a photopolymerizable PS plate may roughly be divided into the following three groups: a) non-aqueous developers mainly comprising organic solvents; b) aqueous developers mainly comprising inorganic alkalis; and c) aqueous developers mainly comprising organic bases.
Among these, those presently used include, for instance, aqueous developers b) and c) at the request of the environmental protection.
Characteristic properties of these two developers will be detailed below. The inorganic alkaline developer b) is characterized in that it usually comprises a silicate and has a pH value in the proximity to 12 in order to hydrophilize the surface of a substrate of the printing plate after the development.
This silicate component is essential for the subsequent hydrophilization treatment or for preventing the non-image area from being contaminated by ink during printing, i.e., for the prevention of scumming.
For instance, there have been known a developer having a pH value of not less than 12 such as those disclosed in Japanese Un-Examined Patent Publication (hereunder referred to as xe2x80x9cJ.P. KOKAIxe2x80x9d) No. Hei 8-248643 and a developer having a pH value of not more than 12 such as those disclosed in J.P. KOKAI No. Hei 11-65129. However, the former or the developer having a pH value of not less than 12 is liable to dissolve aluminum currently used as a substrate for the printing plate and, in particular, in case of a dot (small dot) occupying a small image area, it may dissolve the aluminum substrate immediately below the image area due to the so-called side etching phenomenon and this results in a phenomenon in which small dots are removed from the substrate (skipping of small dots) during printing. In other words, this developer suffers from such a problem that it may significantly deteriorate the printing durability of the resulting printing plate.
Moreover, the latter or the developer having a pH value of not more than 12 is excellent in the foregoing printing durability and an ability of preventing scumming, but it is liable to cause reduction of the pH value when continuing the developing treatment over a long period of time due to, for instance, the effect of carbon dioxide present in the air and the developer causes separation of a silicate during the development and this in turn leads to the occurrence of such a novel problem that it never allows any stable development.
In addition to the foregoing, J.P. KOKAI Nos. Sho 61-109052, 2000-81711 and Hei 11-65126 and West German Patent No. 1,984,605 disclose developers free of any silicate, but all of them were not only inferior in the prevention of scumming to those comprising silicates, but also could simultaneously satisfy the requirements for the printing durability and the prevention of scumming, only with great difficulty.
On the other hand, there have been known, as the organic base-containing developer c), for instance, those each comprising an organic amine such as ethanolamine and an alcoholic organic solvent such as benzyl alcohol as an auxiliary developer. In this case, however, such a developer undoubtedly has a low pH value on the order of 10 and therefore, it is not susceptible to the influence of carbon dioxide and is excellent in the processing stability. However, it also suffers from problems in that it is not only disadvantageous in the hydrophilization of the substrate, but also has an extremely high ability of penetration into the image area, that it adversely affects, in particular, small dots and that it is accompanied by the deterioration of the image-forming ability or such a phenomenon that small dots are removed from the substrate during development.
In other words, under the existing circumstances, there is not proposed any developer suitably used for developing the photopolymerizable PS plate, while taking into consideration, for instance, the image-forming ability, prevention of any scumming and printing durability as well as the processing stability.
Regarding the composition of the developer, it has been known that factors such as the presence of a silicate, the level of pH and the presence of an inorganic or organic alkali may exert a great influence on the developing phenomenon, but any combination thereof conventionally proposed has never permitted the solution of the foregoing problems at all. In particular, when a developer has a relatively lower pH value of approximately 12, such developer suffers greater change in a pH value, which change is associated with carbon dioxide gas and alkali consumption by PS plates, and the developer is susceptible to processing amount of PS plates, lag in adding a replenisher and a concentration of carbon dioxide in a room.
With respect to the processing stability, it is quite critical to consider not only the above-mentioned problem on the silicate-separation associated with the reduction of pH, but also stabilization of pH value of the developer in a relatively lower pH range. In other words, there has been desired for the development of a developer for a photopolymerizable PS plate, which can satisfy the requirements for the foregoing image-forming ability, for both the prevention of scumming and the printing durability and for the processing stability. In addition, in view of improving the prevention of scumming, a desensitizing treatment after a development is contributory thereto and it is necessary to ensure a sufficient volume of a coating by the desensitizing treatment.
Accordingly, it is an object of the present invention to provide a method for preparing a lithographic printing plate, which can satisfy both the requirements for the prevention of scumming and the printing durability on the resultant printing plate without impairing the image-forming ability, and which can improve the processing stability which is associated with a pH change of the developer.
The inventors of this invention have conducted various studies, recognized that the lower pH value of a developer is advantageous so as to avoid the impairment of the image-forming ability of the developer, while searched for suppressing a pH change of the developer, which change is associated with alkali consumption, and then found that an inclusion of a compound having a dissociation constant pka of from 10 to 13 into a developer is effective in suppressing the pH change of the developer, and the development performance is sufficiently exhibited and assured, and the scumming is favorably eliminated, even if the developer has a relatively lower pH value.
Accordingly, the present invention relates to a method for preparing a lithographic printing plate comprising the steps of imagewise exposing, to light, a presensitized plate for use in making a lithographic printing plate, which comprises a grained and anodized substrate provided thereon with a photopolymerizable light-sensitive layer containing a compound having at least one addition-polymerizable ethylenically unsaturated double bond and a titanocene type initiator; and then developing the light-exposed presensitized plate using a developer which comprises a surfactant and a weak acid or a salt thereof having a dissociation constant pka ranging from 10 to 13, and has a pH value ranging from 11.5 to 12.8.
In one embodiment of the method according to the present invention, the light-exposed plate is washed with water, said plate is then developed with the developer and washed with water again, and a desensitizing solution is applied to the surface of the plate with a coating amount of not less than 0.02 g/m2 on a dry basis, said desensitizing solution comprising (a) gum arabic in amount of 0 to 3% by weight and (b) a modified starch in amount of 5 to 30% by weight. In other words, the present invention relates to a method for preparing a lithographic printing plate comprising the steps of imagewise exposing, to light, a presensitized plate for use in making a lithographic printing plate, which comprises a grained and anodized substrate provided thereon with a photopolymerizable light-sensitive layer containing a compound having at least one addition-polymerizable ethylenically unsaturated double bond and a titanocene type initiator; washing the light-exposed plate with water; developing said plate using the foregoing developer; washing the developed plate with water; and then applying a desensitizing solution to the surface of the plate with a coating amount of not less than 0.02 g/m2 on a dry basis, said desensitizing solution comprising (a) gum arabic in amount of 0 to 3% by weight and (b) a modified starch in amount of 5 to 30% by weight.
According to a preferred embodiment of the present invention, the developer used in the method comprises as the surfactant, at least one member selected from the group consisting of nonionic surfactants and anionic surfactants.
According to a further embodiment of the present invention, the developer used in the method comprises a chelating agent for divalent metals. According to a further embodiment of the present invention, the developer used in the method comprises carbonic acid or a carbonate. According to a still further embodiment of the present invention, the developer used in the method has a conductivity of from 3 to 40 mS/cm.
In order to more sufficiently achieve the development performance and prevention of scumming, the developer used in the present invention may comprise a specific compound. Thus, according to an embodiment of present invention, the developer used in the method further comprises at least one member selected from the group consisting of
a compound having a carboxyl group or a salt thereof, represented by the following general formula (1): Rxe2x80x94Arxe2x80x94(COOM)n 
wherein R represents a hydrogen atom or a linear or branched alkyl group having carbon atoms of from 1 to 20, Ar represents an aromatic ring, M represents a hydrogen atom, monovalent metal or an ammonium group, and n represents 1 or 2,
a compound represented by the following general formula (2): (R1)(R2)Nxe2x80x94Xxe2x80x94OH
wherein R1 and R2 each represents an organic residue, and X represents a divalent connecting group,
water-soluble sulfite salts, alkaline-soluble mercapto compounds and thioether compounds.
Furthermore, in a preferred embodiment of the present invention, the light-sensitive layer has an acid value of not more than 1.0 meq/g and more specifically, the light-sensitive layer comprises a compound having an acid group whose pKa value is not more than 9 and the light-sensitive layer has an acid value ranging from 0.20 to 0.60 meq/g.
In another preferred embodiment of the plate-making method according to the present invention, the imagewise light-exposed plate is developed in such a manner that the rate of the development of the non-exposed area is not less than 0.05 xcexcm/s and the rate of the developer penetrating into the exposed area is not more than 0.1 xcexcm/s.
As a still further preferred embodiment of the plate-making method according to the present invention, the substrate of the presensitized plate is an anodized aluminum substrate provided thereon with an organic compound carrying a phosphorus atom-containing acidic group or an organic silicone compound.